The present invention relates to optical information storage media and particularly to thin silver halide-containing films in which information can be optically stored in permanent fashion using ultraviolet light and without any requirement for image development.
The films incorporated in the present media are related in composition to the films described in our concurrently filed co-pending patent application, Ser. No. 086,829, disclosing electron beam sensitive films for optical mask fabrication or the like.
The use of evaporated binder-free silver halide layers as photographic media has long been known. A good summary of the patent literature in this field is found in the U.S. Defensive Publication of Maskasky, T966,003 of Jan. 3, 1978. U.S. Pat. Nos. 2,945,771 to Mansfeld and 3,219,448 to LuValle et al. describe methods by which such films may be deposited on substrates such as glass or plastic, while a detailed discussion of the deposition, processing and performance of evaporated silver bromide films is provided by A. Shepp et al. in "Evaporated Silver Bromide as a Photographic Recording Medium", Photographic Science and Engineering, 11, (5), pp. 316-321 (1967).
U.S. Pat. Nos. 3,219,452 to Hartouni, 3,368,895 to Matejic et al., and 3,658,540 to Malinowski describe materials and/or methods which have been employed to sensitize such photographic films, in order to enhance the latent image formation or chemical developability thereof. In general, binder-free photographic films of the kind described in the above-cited literature are adapted for use in a conventional photographic mode, i.e., a mode wherein the steps of latent image formation by exposure to light and image development by chemical means are required to provide an image of suitable optical density.
Binder-free silver halide based compositions have also been employed in photochromic films, which are films typically exhibiting the properties of visible darkening on exposure to actinic radiation (e.g., short wavelength visible or ultraviolet light) and fading to the original state in the absence thereof. U.S. Pat. No. 3,512,869 to Plumat et al. describes photochromic films incorporating evaporated silver halides or the halides of other metals, which films darken in sunlight and fade in darkness. These films may be catalyzed with copper, cadmium or nickel halides to make them more sensitive to yellow or red visible light, if desired. U.S. Pat. No. 3,875,321 to Gliemeroth and French Pat. No. 2,236,196 are additional patents disclosing reversibly darkenable photochromic films, while in the Soviet Journal of Optical Technology, pp. 117-118 (February 1972), A. F. Perveyev et al. describe AgCl-CuCl photochromic coatings.
Generally, rapid darkening in the presence of light and fast fading in the absence thereof are the properties most desired in photochromic films. Hence such films are not suitable optical information storage media because they do not provide a permanent record of the darkening or fading processes.
A photosensitive film which could exhibit efficient, irreversible darkening at low darkening energy levels without any need for chemical development could be advantageously employed as a digital optical memory material, or as a microfilm medium for direct image recording. Good resolution could be provided if such a film could be made to exhibit acceptable contrast at low film thicknesses, and high signal-to-noise ratios could be obtained if a reading wavelength different from the recording wavelength existed for for the film at which it would be highly insensitive to optical alteration by the reading light.